1. Field of the Invention
The invention relates to a hydrostatic machine.
2. Discussion of the Prior Art
A hydrostatic machine of such kinds is described in DE 44 23 023 A1. In the case of this previously-known hydrostatic machine, the sleeves, in order to avoid piston seizure, are cooled with a cooling flow of the hydraulic fluid, which flows during operation through a respective cooling channel, which roughly radially extends outwards from the leakage chamber in a central hole of the cylinder block and opens out into the leakage chamber outside the cylinder block. The respective cooling channel extends through an inner annular groove in the wall of the sleeve hole seating the associated sleeve or through an inner annular groove in the inner surface of the sleeve. This previously-known cooling method worked satisfactorily in practice, but it is complex since each sleeve must be connected to the radial cooling channel, for the purpose of which annular grooves have to be machined in the sleeves and radial cooling channel sections incorporated in the cylinder block. In addition, this method depends on correctly functioning cooling circuits. If even one cooling channel becomes blocked, unwanted piston seizure can be expected, because the cooling is interrupted or at least reduced.
The danger of piston seizure is caused by increased heating in the region of the sleeves, which develops due to high friction energy from the sliding friction generated between the pistons and the sleeves. The friction energy increases when relative velocities between the pistons and the sleeves become greater. The relative velocity is dependent on the stroke length of the pistons and the speed of revolution of the hydrostatic machine, for example the speed of revolution of a cylinder block formed as a rotatable cylinder drum. At the same time, the greatest friction energy is generated in each case, if the cylinder block is formed by a rotatable cylinder drum, in the region of the centre of gravity of the pistons, when the cylinder drum rotates at high speed of revolution. If the cylinder block is not rotatable or if the cylinder drum rotates at low speed of revolution, the largest friction energy is generated in the region of a radial component of the piston force pointing in the direction of rotation.
Although DE 198 37 647 A1 describes an axial piston machine with sleeves, which according to FIG. 5 have in their longitudinal centre an annular groove in the form of a narrow groove in their outer surface, it is the purpose of this previously known configuration to obtain an annular bulge of the sleeve radially directed inwards by the operating pressure.